In gas turbine engines it is conventional to arrange the various components from one end in the order of low pressure compressor, high pressure compressor, high pressure turbine and low pressure turbine. Often, the low pressure compressor and low pressure turbine rotors are mounted on a low pressure shaft, with an electric generator secured to that shaft. The high pressure compressor rotor and high pressure turbine rotor are secured to a high pressure shaft.
The high pressure shaft has a hollow bore with the low pressure shaft passing concentrically therethrough. The diameter of the low pressure shaft is therefore limited by the requirement that it pass through the high pressure shaft. This makes it difficult to design the shaft with sufficient strength to deal with a shorted generator which can triple the normal torque on the shaft. Also, a long low pressure shaft of relatively small diameter is likely to have critical frequency problems. More than two bearings are therefore usually required.
In conventional engines the low pressure shaft thrust bearing usually is at the compressor end because axial clearances are more critical between the compressor vanes and blades than those of the turbine. The location of the thrust bearing near the compressor limits the differential expansion between the stator and the rotors. The connected generator usually is at the turbine end with the starter equipment at the cold end. Therefore, there is significant low pressure shaft growth toward the coupling to the generator.
The turbine stator is fixed at the end near the generator to minimize the differential expansion between the generator and the gas turbine engine. In the conventional engine this is the turbine end. Therefore, the growth of the low pressure shaft is opposite that of the stator causing large differential axial growth between the stator vanes and the rotor blades.
Also, the coupling to the generator which is located at the turbine end is subject to the high temperatures within the turbine exhaust diffuser tunnel.